This invention relates to perforating systems for perforation of oil well casings and more particularly to apparatus for selectively and separately firing individual shaped charges of a perforating system.
In performing completion operations in a well, it is often necessary to selectively activate electrically detonated explosive devices, so that a number of completion operations at separate well depths may be performed with a single trip into a well.
For example, one of the steps in completing oil and gas wells is perforating the casing to allow entry of the oil or gas. In some wells the oil or gas bearing formation is continuous. The casings in these wells are perforated with one or more guns until the entire productive zone is opened.
The productive formation is not always continuous in all wells. There may be wells that have non-productive streaks in the oil-bearing zone and it would not be desirable to perforate these intervals. Multiple short gun runs, blanked-off shots of long guns, and spacers between guns have all been used to selectively perforate the productive zones in these wells. A better solution is a multi-gun tool where the operator can selectively fire each gun separately.
Existing selectively fired perforating systems of which U.S. Pat. Nos. 3,246,707, 3,246,708 and 3,860,865 are representative prior art, consist of several guns connected in series with down-the-hole switches or switch subs. The switches in these subs are such that the bottom gun is set to fire first. The blast from this first gun switches the first switch sub and the second gun is armed. In a like manner the blast from the second gun will arm the third gun, and so on.
Selective fired guns have been used for many years and a number of problems have been encountered. The selected gun may fail to fire because of electrical circuit failures, including shorts or opens in the wire line or in the down-hole circuit. At times the wrong gun or guns may become armed due to an electrical or mechanical failure. If the wrong gun is fired, the well may be perforated in the wrong zone and expensive repairs, such as cementing, may be required. If any gun fails to fire for any reason, the gun above will not be armed and the firing sequence is stopped. As a result the guns must be pulled out of the well for repair or replacement.
The disclosure of U.S. Pat. No. 3,860,865 cited above is hereby included herein by reference.